An L-band high-power relativistic backward wave oscillator is designed. In the simulation, microwaves centered at 1.6 GHz are generated, with the power of 3.6 GW and the efficiency of 40%. In the preliminary experiment, the pulse duration of the device was only 45 ns, presenting a pulse-shortening phenomenon. Through the 3D particle-in-cell simulation analysis, it was found that the accelerating diode resonances significantly impact the operation of the L-band high power relativistic backward wave oscillator, and the resonance of a TE11 mode in the accelerating diode played the primary role in the pulse shortening. Moreover, we found that choosing the appropriate distance between the cathode baffle and the end of the annular cathode is beneficial to effectively suppress the starting oscillation of the parasitic TE11 mode. In the improved experiment, we changed the distance between the cathode baffle and the end of the annular cathode from previous 5.4 to 4.6 cm. Eventually, when the diode voltage is 650 kV and the diode current is 14 kA, microwaves centered at 1.58 GHz are generated with the power of 3.3 GW, the efficiency of 36%, and the pulse duration above 104 ns.
With an increase in the input power of a low-magnetic field and high-efficiency transit-time oscillator, the deceleration electric field in the single-gap extractor increases sharply to output a higher-power microwave, resulting in radio frequency breakdown on the surface of the single-gap extractor. Thus, a doublegap extractor with an operating mode of π mode is introduced to reduce the electric field in this type of device. The double-gap extractor can extract electron energy in two stages and lengthen the deceleration distance. Therefore, the electron beam loses less energy at the same deceleration distance in the double-gap extractor, and a weaker deceleration electric field is generated in the double-gap extractor. Additionally, the modulation electric field in the device with a double-gap extractor is stronger for modulating electrons and can further increase the fundamental harmonic current, resulting in higher extraction efficiency. In simulations, the output power is 6.3 GW and the maximum electric field strength is 1.06 MV/cm in the initial model with a single-gap extractor. When the single-gap extractor is replaced by the double-gap extractor, the output power increases to 6.5 GW with a conversion efficiency of 40% and a guiding magnetic field of 0.6 T. The maximum electric field strength decreases to 817 kV/cm, indicating an enhancement in the power capacity of the device.INDEX TERMS High-power microwaves, transit-time oscillator, improved power capacity, deceleration electric field.
An improved X-band coaxial transit time oscillator is proposed in this paper. First, the device uses a non-uniform three-gap modulation cavity to improve the clustering of electron beams under a low magnetic field and to increase the depth of the fundamental current modulation. The operating mode of the modulation cavity is the 2π/3 mode of TM01. Second, the dual-cavity extraction structure works in π mode, which can continuously and intensively extract the electron beam energy. The physical characteristics of the device are studied through simulation and experiment. The experimental results indicate that the high power microwave with a frequency of 8.38 GHz and a power of 1.78 GW is generated when the diode voltage is 520 kV and the guiding magnetic field is 0.65 T, yielding a pulse width of 27 ns and an efficiency of 32%.
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